Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train may include input and output shafts, first to fourth planetary gear sets having first to third, fourth to sixth, seventh to ninth, and tenth to twelfth elements, a first shaft fixedly connected to the fourth and tenth elements, and selectively connectable to the input shaft, a second shaft fixedly connected to the third element, and selectively connectable to the input shaft, a third shaft fixedly connected to the eleventh element the output shaft, a fourth shaft fixedly connected to the first, sixth, and ninth elements, a fifth shaft fixedly connected to the eighth and twelfth elements, and a plurality of shafts each of which is selectively connectable to the transmission housing and fixedly connected to an element of the first, second and third planetary gear sets which is not fixedly connected to any of the first to seventh shafts.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2017-0145211 filed on Nov. 2, 2017, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an automatic transmission for a vehicle.

Description of Related Art

Research on realizing more shift-stages of an automatic transmission are undertaken to achieve enhancement of fuel consumption and better drivability, and recently, increase of oil price is triggering a hard competition in enhancing fuel consumption of a vehicle.

In this sense, research on an engine has been undertaken to achieve weight reduction and to enhance fuel consumption by so-called downsizing research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages.

To achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.

Therefore, to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by fewer number of parts.

In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission facilitating more shift stages is under investigation.

An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to seven engagement elements (frictional elements), and may easily become lengthy, deteriorating installability.

In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and may not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle facilitating at least ten forward speeds, providing better performance and fuel efficiency of a vehicle.

A planetary gear train according to an exemplary embodiment of the present invention may include an input shaft receiving an engine torque, an output shaft outputting a shifted torque, a first planetary gear set having first, second and third rotation elements, a second planetary gear set having fourth, fifth, and sixth rotation elements, a third planetary gear set having seventh, eighth, and ninth rotation elements, and a fourth planetary gear set having tenth, eleventh, and twelfth rotation elements. An exemplary planetary gear train may further include a first shaft fixedly connected to the fourth rotation element and the tenth rotation element, and selectively connectable to the input shaft, a second shaft fixedly connected to the third rotation element, and selectively connectable to the input shaft, a third shaft fixedly connected to the eleventh rotation element, and fixedly connected to the output shaft, a fourth shaft fixedly connected to the first rotation element, the sixth rotation element, and the ninth rotation element, a fifth shaft fixedly connected to the eighth rotation element and the twelfth rotation element, and a plurality of shafts each of which is selectively connectable to the transmission housing and fixedly connected to a rotation element of the first, second and third planetary gear sets which is not fixedly connected to any of the first to seventh shafts.

The plurality of shafts may include a sixth shaft fixedly connected to the second rotation element and the fifth rotation element, and selectively connectable to the transmission housing, and a seventh shaft fixedly connected to the seventh rotation element, and selectively connectable to the transmission housing. The input shaft and the first shaft, the input shaft and the second shaft, the third shaft and the fourth shaft, and the fifth shaft and the seventh shaft may be selectively interconnected to each other, respectively.

An exemplary planetary gear grain may further include four clutches each selectively connecting a corresponding pair among the input shaft, the output shaft, and the first to seventh shafts, and two brakes selectively connecting the sixth shaft and the seventh shaft to the transmission housing respectively.

The four clutches may include a first clutch disposed between the input shaft and the first shaft, a second clutch disposed between the input shaft and the second shaft, a third clutch disposed between the third shaft and the fourth shaft, and a fourth clutch disposed between the fifth shaft and the seventh shaft,

The two brakes may include a first brake disposed between the sixth shaft and the transmission housing, and a second brake disposed between the seventh shaft and the transmission housing.

The first planetary gear set may be a single pinion planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as the first, second and third rotation elements. The second planetary gear set may be a single pinion planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as the fourth, fifth, and sixth rotation elements. The third planetary gear set may be a double pinion planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as the seventh, eighth, and ninth rotation elements. The fourth planetary gear set may be a single pinion planetary gear set having a fourth sun gear, a fourth planet carrier, and a fourth ring gear as the tenth, eleventh, and twelfth rotation elements.

According to an exemplary embodiment of the present invention, shift-stages of at least ten forward speeds and at least one reverse speed may be realized by combination of four planetary gear sets of simple planetary gear sets and six engagement elements.

Furthermore, a planetary gear train according to an exemplary embodiment of the present invention may substantially improve driving stability by realizing shift-stages appropriate for rotation speed of an engine due to multi-stages of an automatic transmission.

Furthermore, a planetary gear train according to an exemplary embodiment of the present invention maximize engine driving efficiency by multi-stages of an automatic transmission, and may improve power delivery performance and fuel consumption.

Furthermore, effects that can be obtained or expected from exemplary embodiments of the present invention are directly or suggestively described in the following detailed description. That is, various effects expected from exemplary embodiments of the present invention will be described in the following detailed description.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 2 is an operational chart for respective control elements at respective shift-stages applicable to a planetary gear train according to an exemplary embodiment of the present invention.

It may be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in portion by the particularly intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

Hereinafter, an exemplary embodiment of the present invention will be described more specifically with reference to drawings.

The drawings and description are to be regarded as illustrative in nature and not restrictive, and like reference numerals designate like elements throughout the specification.

In the following description, dividing names of components into first, second, and the like is to divide the names because the names of the components are the same as each other and an order thereof is not particularly limited.

FIG. 1 is a schematic diagram of a planetary gear train according to an exemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplary embodiment of the present invention includes first, second, third and fourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on a same axis, an input shaft IS, an output shaft OS, seven shafts TM1 to TM7 interconnecting rotation elements of the first, second, third and fourth planetary gear sets PG1, PG2, PG3, and PG4, engagement elements of four clutches C1 to C4 and two brakes B1 and B2, and a transmission housing H.

Torque input from the input shaft IS is shifted by cooperative operation of the first, second, third and fourth planetary gear sets PG1, PG2, PG3, and PG4, and then output through the output shaft OS.

In an exemplary embodiment of the present invention, the planetary gear sets are disposed in the order of the first, the second, and the third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, from an engine side.

The input shaft IS is an input member and may receive a torque from a crankshaft of an engine through a torque converter.

The output shaft OS is an output element disposed on a same axis with the input shaft IS, and outputs a shifted driving torque to a driveshaft through a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear set, and includes a first sun gear S1, a first planet carrier PC1 rotatably supporting a plurality of first pinion gears P1 externally gear-meshed with the first sun gear S1, and a first ring gear R1 internally gear-meshed with the plurality of first pinion gears P1 engaged with the first sun gear S1. The first sun gear Si acts as a first rotation element N1, the first planet carrier PC1 acts as a second rotation element N2, and the first ring gear R1 acts as a third rotation element N3.

The second planetary gear set PG2 is a single pinion planetary gear set, and includes a second sun gear S2, a second planet carrier PC2 rotatably supporting a plurality of second pinion gears P2 externally gear-meshed with the second sun gear S2, and a second ring gear R2 internally gear-meshed with the plurality of second pinion gears P2 engaged with the second sun gear S2. The second sun gear S2 acts as a fourth rotation element N4, the second planet carrier PC2 acts as a fifth rotation element N5, and the second ring gear R2 acts as a sixth rotation element N6.

The third planetary gear set PG3 is a double pinion planetary gear set, and includes a third sun gear S3, a third planet carrier PC3 rotatably supporting a plurality of third pinion gears P3 in pairs externally gear-meshed with the third sun gear S3, and a third ring gear R3 internally gear-meshed with the plurality of third pinion gears P3 engaged with the third sun gear S3. The third sun gear S3 acts as a seventh rotation element N7, the third planet carrier PC3 acts as an eighth rotation element N8, and the third ring gear R3 acts as a ninth rotation element N9.

The fourth planetary gear set PG4 is a single pinion planetary gear set, and includes a fourth sun gear S4, a fourth planet carrier PC4 rotatably supporting a plurality of fourth pinion gears P4 externally gear-meshed with the fourth sun gear S4, and a fourth ring gear R4 internally gear-meshed with the plurality of fourth pinion gears P4 engaged with the fourth sun gear S4. The fourth sun gear S4 acts as tenth rotation element N10, the fourth planet carrier PC4 acts as an eleventh rotation element N11, and the fourth ring gear R4 acts as a twelfth rotation element N12

In the arrangement of the first, second, third and fourth planetary gear sets PG1, PG2, PG3, and PG4, the first rotation element N1 is fixedly connected to the sixth rotation element N6 and the ninth rotation element N9, the second rotation element N2 is fixedly connected to the fifth rotation element N5, the fourth rotation element N4 is fixedly connected to the tenth rotation element N10, and the eighth rotation element N8 is fixedly connected to the twelfth rotation element N12, forming seven shafts TM1 to TM7.

The seven shafts TM1 to TM7 are hereinafter described more specifically.

The first shaft TM1 is fixedly connected to the fourth rotation element (N4; second sun gear S4) and the tenth rotation element N10 (fourth sun gear S4), and selectively connectable to the input shaft IS selectively acting as an input element.

The second shaft TM2 is fixedly connected to the third rotation element N3 (first ring gear R1), and selectively connectable to the input shaft IS selectively acting as an input element.

The third shaft TM3 is fixedly connected to the eleventh rotation element N11 (fourth planet carrier PC4), and fixedly connected to the output shaft OS always acting as an output element.

The fourth shaft TM4 is fixedly connected to the first rotation element N1 (first sun gear Si), the sixth rotation element N6 (second ring gear R2), and the ninth rotation element N9 (third ring gear R3).

The fifth shaft TM5 is fixedly connected to the eighth rotation element N8 (third planet carrier PC3) and the twelfth rotation element N12 (fourth ring gear R4).

The sixth shaft TM6 is fixedly connected to the second rotation element N2 (first planet carrier PC1) and the fifth rotation element N5 (second planet carrier PC2), and selectively connectable to the transmission housing H, selectively acting as a fixed element.

The seventh shaft TM7 is fixedly connected to the seventh rotation element N7 (third sun gear S3), and selectively connectable to the transmission housing H, selectively acting as a fixed element.

Each of the seven shafts TM1 to TM7 may be a rotation member that fixedly interconnects the input and output shafts and rotation elements of the planetary gear sets PG1, PG2, PG3, and PG4, or may be a rotation member that selectively interconnects a rotation element to the transmission housing H, or may be a fixed member fixed to the transmission housing H.

In the disclosure, when two or more members are described to be “fixedly connected”, where the member may be any of a shaft, an input shaft, an output shaft, a rotation member, and a transmission housing, it means that the fixedly connected members always rotate at a same speed.

When two or more members are described to be “selectively connectable” by an engagement element, it means that the selectively connectable members rotates separately when the engagement element is not engaged, and rotates at a same speed when the engagement element is engaged. It may be understood that in the case that a member is “selectively connectable” with a transmission housing by an engagement element, the member may be stationary when the engagement element is engaged.

The first shaft TM1 and the second shaft TM2 are selectively connectable to the input shaft IS respectively, the third shaft TM3 is selectively connectable to the fourth shaft TM4, and the fifth shaft TM5 is selectively connectable to the seventh shaft TM7.

The sixth shaft TM6 and the seventh shaft TM7 are selectively connectable to the transmission housing H, selectively acting as a fixed element.

The engagement elements of four clutches C1, C2, C3, and C4 are disposed between the seven shafts TM1 to TM7, the input shaft IS, and the output shaft OS, to form selective connections.

The seven shafts TM1 to TM7 may be selectively connectable to the transmission housing H, by control elements of two brakes B1 and B2.

The six engagement elements of the four clutches C1 to C4 and the two brakes B1 and B2 are disposed as follows.

The first clutch C1 is disposed between the input shaft IS and the first shaft TM1, and selectively connects the input shaft IS and the first shaft TM1, controlling power delivery therebetween.

The second clutch C2 is disposed between the input shaft IS and the second shaft TM2, and selectively connects the input shaft IS and the second shaft TM2, controlling power delivery therebetween.

The third clutch C3 is disposed between the third shaft TM3 and the fourth shaft TM4, and selectively connects the third shaft TM3 and the fourth shaft TM4, controlling power delivery therebetween.

The fourth clutch C4 is disposed between the fifth shaft TM5 and the seventh shaft TM7, and selectively connects the fifth shaft TM5 and the seventh shaft TM7, controlling power delivery therebetween.

The first brake B1 is disposed between the sixth shaft TM6 and the transmission housing H, and selectively connects the sixth shaft TM6 to the transmission housing H.

The second brake B2 is disposed between the seventh shaft TM7 and the transmission housing H, and selectively connects the seventh shaft TM7 to the transmission housing H.

The engagement elements of the first, second, third and fourth clutches C1, C2, C3, and C4 and the first and second brakes B1 and B2 may be realized as multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure, however, it may not be understood to be limited thereto, since various other configuration that are electrically controllable may be available.

FIG. 2 is an operational chart for respective control elements at respective shift-stages applicable to a planetary gear train according to an exemplary embodiment of the present invention.

Referring to FIG. 2, a planetary gear train according to an exemplary embodiment of the present invention realizes shifting between the tenth forward speed and one reverse speed by operating three elements among the engagement element of the first, second, third and fourth clutches C1, C2, C3, and C4 and the first and second brakes B1 and B2.

In the first forward speed D1, the first and fourth clutches C1 and C4 and the first brake B1 are simultaneously operated.

As a result, the first shaft TM1 is connected to the input shaft IS by the operation of the first clutch C1, and the fifth shaft TM5 is connected to the seventh shaft TM7 by the operation of the fourth clutch C4. In the present state, the input torque is input to the first shaft TM1.

Furthermore, the sixth shaft TM6 acts as a fixed element by the operation of the first brake B1, realizing the first forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the second forward speed D2, the first clutch C1 and the first and second brakes B1 and B2 are simultaneously operated.

As a result, the first shaft TM1 is connected to the input shaft IS by the operation of the first clutch C1. In the present state, the input torque is input to the first shaft TM1.

In the present state, the sixth shaft TM6 and the seventh shaft TM7 respectively operate as fixed elements by the operation of the first and second brakes B1 and B2, realizing the second forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the third forward speed D3, the first and fourth clutches C1 and C4 and the second brake B2 are simultaneously operated.

As a result, the first shaft TM1 is connected to the input shaft IS by the operation of the first clutch C1, and the fifth shaft TM5 is connected to the seventh shaft TM7 by the operation of the fourth clutch C4. In the present state, the input torque is input to the first shaft TM1.

Furthermore, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, realizing the third forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the fourth forward speed D4, the first and third clutches C1 and C3 and the second brake B2 are simultaneously operated.

As a result, the first shaft TM1 is connected to the input shaft IS by the operation of the first clutch C1, and the third shaft TM3 is connected to the fourth shaft TM4 by the operation of the third clutch C3. In the present state, the input torque is input to the first shaft TM1.

Furthermore, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, realizing the fourth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the fifth forward speed D5, the first clutch and second clutch C1 and C2 and the second brake B2 are simultaneously operated.

As a result, the first shaft TM1 and the second shaft TM2 are connected to the input shaft by the operation of the first clutch and second clutch C1 and C2. In the present state, the input torque is input to the first shaft TM1 and the second shaft TM2.

Furthermore, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, realizing the fifth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the sixth forward speed D6, the second and third clutches C2 and C3 and the second brake B2 are simultaneously operated.

As a result, the second shaft TM2 is connected to the input shaft IS by the operation of the second clutch C2, and the third shaft TM3 is connected to the fourth shaft TM4 by the operation of the third clutch C3. In the present state, the input torque is input to the second shaft TM2.

Furthermore, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, realizing the sixth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the seventh forward speed D7, the second, third, and fourth clutches C2, C3, and C4 are simultaneously operated.

As a result, the second shaft TM2 is connected to the input shaft IS by the operation of the second clutch C2, the third shaft TM3 is connected to the fourth shaft TM4 by the operation of the third clutch C3, and the fifth shaft TM5 is connected to the seventh shaft TM7 by the operation of the fourth clutch C4.

As such, the first, second, third and fourth planetary gear sets PG1, PG2, PG3, and PG4 integrally rotate, and a torque is input to the second shaft TM2, realizing the seventh forward speed where a torque is output as inputted, and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the eighth forward speed D8, the second and fourth clutches C2 and C4 and the second brake B2 are simultaneously operated.

As a result, the second shaft TM2 is connected to the input shaft IS by the operation of the second clutch C2, and the fifth shaft TM5 is connected to the seventh shaft TM7 by the operation of the fourth clutch C4. In the present state, the input torque is input to the second shaft TM2.

Furthermore, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, realizing the eighth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the ninth forward speed D9, the second and fourth clutches C2 and C4 and the first brake B1 are simultaneously operated.

As a result, the second shaft TM2 is connected to the input shaft IS by the operation of the second clutch C2, and the fifth shaft TM5 is connected to the seventh shaft TM7 by the operation of the fourth clutch C4. In the present state, the input torque is input to the second shaft TM2.

Furthermore, the sixth shaft TM6 acts as a fixed element by the operation of the first brake B1, realizing the ninth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM2.

In the tenth forward speed D10, the second clutch C2 and the first and second brakes B1 and B2 are simultaneously operated.

As a result, the second shaft TM2 is connected to the input shaft IS by the operation of the second clutch C2. In the present state, the input torque is input to the second shaft TM2.

In the present state, the sixth shaft TM6 and the seventh shaft TM7 respectively act as fixed elements by the operation of the first and second brakes B1 and B2, realizing the tenth forward speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

In the reverse speed REV, the first and third clutches C1 and C3 and the first brake B1 are simultaneously operated.

As a result, the first shaft TM1 is connected to the input shaft IS by the operation of the first clutch C1, and the third shaft TM3 is connected to the fourth shaft TM4 by the operation of the third clutch C3. In the present state, the input torque is input to the first shaft TM1.

Furthermore, the sixth shaft TM6 acts as a fixed element by the operation of the first brake B 1, realizing the reverse speed and outputting a shifted torque to the output shaft OS connected to the third shaft TM3.

As described above, a planetary gear train according to an exemplary embodiment of the present invention may realize the tenth forward speed and one reverse speed by operating four planetary gear set PG1, PG2, PG3, and PG4 by controlling four clutches C1, C2, C3, and C4 and two brakes B1 and B2.

Furthermore, a planetary gear train according to an exemplary embodiment of the present invention may improve driving stability by realizing shift-stages appropriate for rotation speed of an engine due to multi-stages of an automatic transmission.

Furthermore, a planetary gear train according to an exemplary embodiment of the present invention maximize engine driving efficiency by multi-stages of an automatic transmission, and may improve power delivery performance and fuel consumption.

For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “internal”, “outer”, “up”, “down”, “upper”, “lower”, “upwards”, “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly”, “outwardly”, “internal”, “external”, “internal”, “outer”, “forwards”, and “backwards” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described to explain certain principles of the invention and their practical application, to enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A planetary gear set of an automatic transmission for a vehicle, comprising: an input shaft receiving an engine torque; an output shaft outputting a shifted torque; a first planetary gear set having a first rotation element, a second rotation element, and a third rotation element; a second planetary gear set having a fourth rotation element, a fifth rotation element, and a sixth rotation element; a third planetary gear set having a seventh rotation element, an eighth rotation element, and a ninth rotation element; a fourth planetary gear set having a tenth rotation element, an eleventh rotation element, and a twelfth rotation element; a first shaft fixedly connected to the fourth rotation element and the tenth rotation element, and selectively connectable to the input shaft; a second shaft fixedly connected to the third rotation element, and selectively connectable to the input shaft; a third shaft fixedly connected to the eleventh rotation element, and fixedly connected to the output shaft; a fourth shaft fixedly connected to the first rotation element, the sixth rotation element, and the ninth rotation element; a fifth shaft fixedly connected to the eighth rotation element and the twelfth rotation element; and a plurality of shafts each of which is selectively connectable to a transmission housing and fixedly connected to a rotation element of the first, second and third planetary gear sets which is not fixedly connected to the first to seventh shafts.
 2. The planetary gear set of claim 1, wherein the plurality of shafts includes: a sixth shaft fixedly connected to the second rotation element and the fifth rotation element, and selectively connectable to the transmission housing; and a seventh shaft fixedly connected to the seventh rotation element, and selectively connectable to the transmission housing, wherein the input shaft is selectively connectable to the first shaft, the input shaft is selectively connectable to the second shaft, the third shaft is selectively connectable to the fourth shaft, and the fifth shaft is selectively connectable to the seventh shaft.
 3. The planetary gear set of claim 2, further including: four clutches each selectively connecting a corresponding pair among the input shaft, the output shaft, and the first to seventh shafts; and two brakes selectively connecting the sixth shaft and the seventh shaft to the transmission housing respectively.
 4. The planetary gear set of claim 3, wherein the four clutches comprise: a first clutch mounted between the input shaft and the first shaft; a second clutch mounted between the input shaft and the second shaft; a third clutch mounted between the third shaft and the fourth shaft; and a fourth clutch mounted between the fifth shaft and the seventh shaft, and wherein the two brakes includes: a first brake mounted between the sixth shaft and the transmission housing; and a second brake mounted between the seventh shaft and the transmission housing.
 5. The planetary gear set of claim 1, wherein the first planetary gear set is a single pinion planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as the first rotation element, the second rotation element, and the third rotation element, respectively; the second planetary gear set is a single pinion planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as the fourth rotation element, the fifth rotation element, and the sixth rotation element, respectively; the third planetary gear set is a double pinion planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as the seventh rotation element, the eighth rotation element, and the ninth rotation element, respectively; and the fourth planetary gear set is a single pinion planetary gear set having a fourth sun gear, a fourth planet carrier, and a fourth ring gear as the tenth rotation element, the eleventh rotation element, and the twelfth rotation element.
 6. A planetary gear set of an automatic transmission for a vehicle, comprising: an input shaft receiving an engine torque; an output shaft outputting a shifted torque; a first planetary gear set having a first rotation element, a second rotation element, and a third rotation element; a second planetary gear set having a fourth rotation element, a fifth rotation element, and a sixth rotation element; a third planetary gear set having a seventh rotation element, an eighth rotation element, and a ninth rotation element; a fourth planetary gear set having a tenth rotation element, an eleventh rotation element, and a twelfth rotation element, wherein the first rotation element is fixedly connected to the sixth rotation element and the ninth rotation element, the second rotation element is fixedly connected to the fifth rotation element, and selectively connectable to a transmission housing, the third rotation element is selectively connectable to the input shaft, the fourth rotation element is fixedly connected to the tenth rotation element, and selectively connectable to the input shaft, the seventh rotation element is selectively connectable to the transmission housing, the eighth rotation element is fixedly connected to the twelfth rotation element, and selectively connectable to the seventh rotation element, and the eleventh rotation element is selectively connectable to the ninth rotation element, and fixedly connected to the output shaft.
 7. The planetary gear set of claim 6, further including: four clutches each selectively connecting a corresponding pair among the input shaft, the output shaft, and the first to twelfth rotation elements; and two brakes selectively connecting the second rotation element and the seventh rotation element to the transmission housing respectively;
 8. The planetary gear set of claim 7, wherein the four clutches comprise: a first clutch mounted between the input shaft and the fourth rotation element; a second clutch mounted between the input shaft and the third rotation element; a third clutch mounted between the ninth rotation element and the eleventh rotation element; and a fourth clutch mounted between the seventh rotation element and the eighth rotation element, wherein the two brakes comprise: a first brake mounted between the second rotation element and the transmission housing; and a second brake mounted between the seventh rotation element and the transmission housing.
 9. The planetary gear set of claim 6, wherein the first planetary gear set is a single pinion planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as the first rotation element, the second rotation element, and the third rotation element, respectively; the second planetary gear set is a single pinion planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as the fourth rotation element, the fifth rotation element, and the sixth rotation element, respectively; the third planetary gear set is a double pinion planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as the seventh rotation element, the eighth rotation element, and the ninth rotation element, respectively; and the fourth planetary gear set is a single pinion planetary gear set having a fourth sun gear, a fourth planet carrier, and a fourth ring gear as the tenth rotation element, the eleventh rotation element, and the twelfth rotation element. 